When the lips have gone, the teeth will feel cold
This indicates that two sides — and by extension their losses and gains — are interrelated
Neutral
依战略形势来看，这两国有唇亡齿寒的利害关系 | Yī zhànlüè xíngshì lái kàn, zhè liǎng guó yǒu chúnwángchǐhán de lìhài guānxì
From a strategic point of view, these two countries' losses and gains are directly related to one another
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在强敌面前，毗邻的小国总是结成联盟，互相支持，以求生存，因为他们懂得唇亡齿寒的这个道理 | Zài qiáng dí miànqián, pílín de xiǎoguó zǒng shì jié chéng liánméng, hùxiāng zhīchí, yǐ qiú shēngcún, yīnwèi tāmen dǒng de chúnwángchǐhán de zhège dàolǐ
In the face of a strong enemy, small, neighboring countries always form an alliance and help one another in order to survive. This is because they understand that their losses and gains depend on one another
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《春秋左传》(Chūn Qiū Zuò Zhuàn), a famous work that discusses political and military affairs during the Spring and Autumn Period (770 - 476 BC). It was written by the famous blind historian 左丘明 (Zuò Qiū Míng) and is also known as 《左传》(Zuò Zhuàn)
During the Spring and Autumn Period (春秋时期 | Chūn Qiú Shí Qī | 770 - 476 BC), the kingdom of 晋国 (Jìn Guó) was determined to defeat the country of 虢国 (Guó Guó). In order to reach 虢国, however, 晋国 needed to pass through the country of 虞国 (Yú Guó), so they reached out to 虞国 and asked to be granted permission.
A 虞国 official named 宫之奇 (Gōng Zhī Qí) vehemently opposed allowing 晋国 to traverse their territory.
"We are neighbors with 虢国, and our fates are closely linked (唇亡齿寒)," he told his ruler. "If they fall to 晋国's hands, 晋国 will attack us next, and we will have a difficult time defending ourselves."
His lord thought that this advice was foolish and fear-mongering, so he granted 晋国 passage. 虢国 soon fell to 晋国's hands, and on the way home, 晋国 attacked and defeated 虞国.